The present invention relates to a system configuration and controls enabling a combined cycle system to transition smoothly from a startup condition initially supplying cold, dry fuel gas to the gas turbine to high load (typically premixed combustion) operation supplying heated moisturized fuel gas to the gas turbine and from operation with heated moisturized fuel to shutdown with dry fuel.
Generally, a combined cycle power plant contains a gas turbine, a steam turbine, a heat recovery system, a fuel superheater and a fuel gas saturator. Dry cold fuel gas is supplied to the fuel gas saturator, where the fuel gas is moisturized. The fuel gas is then heated by the fuel superheater and is supplied to the gas turbine for combustion. The combustion reaction drives the turbine and a generator coupled to the turbine to produce electricity. The exhaust from the gas turbine enters a heat recovery steam generator (HRSG), which utilizes the heat from the exhaust gases to generate steam for use in the steam turbine and heat water for use in the fuel gas saturator and to superheat the fuel gas in the fuel gas superheater. The steam generated expands in the steam turbine for generating power.
Natural gas fired gas turbines with Dry Low NOx (DLN) combustion systems impose strict requirements on the fuel saturation process due to tight fuel specification tolerances for variables, such as fuel composition, heating value and temperature. Typically these systems have at least two operating modes for optimized performance: one occurs from initial ignition or startup through early loading and the other at high load conditions. Minimizing system emissions is critical during operation at high load conditions. Operation at high load uses fully premixed fuel gas where the system is finely tuned for optimal performance and typically has low tolerance for variations in the fuel supply. Variations in the properties of cold, dry fuel gas and heated, moisturized fuel gas are, however, greatly different. As a consequence, the fuel moisturization system must achieve stable operation with heated and moisturized fuel before the combustion system reaches fully premixed operation. A characteristic fuel property important for combustion stability is the Wobbe number defined as follows:       Wobbe    ⁢          xe2x80x83        ⁢    Number    =            Fuel      ⁢              xe2x80x83            ⁢      Lower      ⁢              xe2x80x83            ⁢      Heating      ⁢              xe2x80x83            ⁢      Value      ⁢              xe2x80x83            ⁢              (                  Btu          /          scf                )                            (                  Fuel          ⁢                      xe2x80x83                    ⁢          Temperature          ⁢                      xe2x80x83                    ⁢                      (                          Deg              .                              xe2x80x83                            ⁢              Rankine                        )                    xc3x97          Fuel          ⁢                      xe2x80x83                    ⁢                                    Mol              .                              xe2x80x83                            ⁢              Wt              .                        /            28.96                          )            
Fuel gas saturation has been employed in a number of integrated gasification combined cycle (IGCC) installations over the last two decades. All IGCCs are designed with a backup fuel to increase plant availability, which is otherwise poor due to gasification system needs. Also, since syngas is high in hydrogen content, the syngas combustion system is designed for diffusion operation, which has much higher tolerance to fuel supply Wobbe number variation than the DLN combustion system employed on most modem natural gas fired turbines. Those two features of IGCC combustion systems combine to ease the challenges of bringing the fuel moisturization system on line. In particular, syngas is flared until it has reached design conditions (e.g., composition, purity, temperature and moisture content, etc.), during which time the gas turbine is fired using backup fuel. This decreases the range of operation on syngas and eases both the combustion system design and mode transition challenges. To our knowledge, no gas turbines or gas turbine combined cycle system with a fuel moisturization system has been built and operated without reliance on backup fuel for initial gas turbine firing.
In accordance with a preferred embodiment of the present invention, there is provided a system configuration and associated control sequencing which enables a combined cycle system with fuel gas moisturization to start using cold, dry fuel gas and transition smoothly to heated moisturized fuel gas, preferably prior to entry into premixed combustion operation, without resort to backup fuel or temporary fuel stream flare. Similarly, shutdown and transient conditions are accommodated in the system configuration and its controls.
In a preferred embodiment of this invention, there is provided a process which enables the gas turbine startup on cold, dry gas fuel when heat is not initially available to operate the fuel gas superheater or the fuel moisturization system. The operation of the gas turbine fuel control system transitions smoothly from use of cold, dry fuel gas, through heated dry fuel gas and finally through heated moisturized fuel gas during startup after heat is available to operate the fuel gas superheater and the fuel moisturization a system, but before the unit reaches premixed combustion operation. The gas turbine system does not employ backup fuel or a temporary fuel stream flare during startup.
More particularly, in this preferred embodiment, a saturator fuel gas bypass enables cold, dry fuel gas to enter the superheater without passing through the fuel gas saturator. This bypass permits the fuel gas superheater to heat the fuel gas upon admission of heated water from the IP (intermediate pressure) economizer outlet of the HRSG (heat recovery steam generator) into the superheater. The bypass also enables the fuel gas superheater and the fuel gas saturator to be brought to operating temperature independently of one another and at controlled rates. This independent operation enables greater control and flexibility during startup. Additionally, the fuel gas saturator is isolated from the fuel gas superheater during low load operation. This isolation permits the fuel gas superheater to begin operation prior to addition of moisture to the fuel. Once the saturator water is heated and is pressurized, fuel gas admission to the saturator commences. For example, this may occur at approximately 30% load. Fuel gas admission then ramps up to full fuel gas flow, e.g., at approximately 35% load. When the saturator pressure reaches the dry fuel gas supply pressure, moisturized fuel gas is admitted to the fuel gas superheater for heating en route to the gas turbine.
From the foregoing, it will be appreciated that the system configuration enables the cold, dry fuel gas to initially bypass the saturator, enabling both the fuel superheater and fuel gas saturator to be brought to operating temperature independently and at a controlled rate. Additionally, the saturator is isolated from the fuel superheater during initial and low load operation, enabling the fuel gas superheater to begin operation prior to addition of moisture to the fuel gas. Aspects of the control sequencing to enable smooth transfer from cold, dry fuel gas to heated moisturized fuel gas include heating the cold, dry fuel in the fuel superheater prior to admission of moisturized (saturated) fuel gas to the fuel gas superheater. This enables moisture to be introduced as gradually as desired, while always maintaining adequate fuel superheat. Further, these control sequencing features enable transition from cold, dry fuel gas to heated, moisturized fuel gas before the combustion mode of the gas turbine transfers from diffusion operation to a premixed operation. This enables the gas fuel supply conditions to the gas turbine to be stabilized at rated conditions prior to combustion system operation in a mode that is most sensitive to variations in gas fuel supply conditions.
In another aspect of the present invention, shutdown of the system may be accomplished utilizing essentially a reverse of the startup procedure. Thus, when the plant unloads, e.g., to approximately 25% load, the fuel gas supply to the fuel gas saturator is ramped off over a predetermined time. Water flow to the saturator continues at a minimum flow. Water flow to the fuel gas superheater holds the fuel gas temperature to a desired temperature until the temperature of the fuel gas superheater discharge water exceeds the transfer pump suction temperature by approximately 25xc2x0 F. At that point, the water flow to the superheater is modulated to minimize the temperature differential at the fuel superheater water return connection. The system is thus retained in a state of readiness should the operator elect to reload the plant. Water flow to the fuel gas superheater continues until the gas turbine fuel gas flow is shut off. Water flow to the fuel gas superheater is then shut off and the superheater remains full of water at the same pressure as the HRSG IP economizer. Thus, the shell sides of the fuel gas superheater and the fuel gas saturator are bottled up with dry fuel gas and a mixture of fuel gas and water vapor, respectively, and isolated from one another. Purging is not necessary unless maintenance access is required or if saturated drum water temperature drops below a predetermined temperature, e.g., approximately 70xc2x0 F.
In a preferred embodiment according to the present invention, there is provided in a fuel supply system having a superheater for heating fuel gas for supply to a gas turbine, a saturator for moisturizing fuel gas and supplying moisturized fuel gas to the superheater during high load operation, and a dry fuel gas supply conduit for supplying fuel gas to the superheater and the saturator, a method of controlling the supply of fuel gas to the gas turbine during startup comprising the steps of (a) heating cold, dry fuel gas in the superheater and supplying the heated dry fuel gas from the superheater to the gas turbine before admitting moisturized fuel gas to the superheater.
In a further preferred embodiment according to the present invention, there is provided in a combined cycle system having a gas turbine, a heat recovery steam generator for recovering heat from gas turbine exhaust, a fuel superheater for heating fuel gas and supplying the heated fuel gas to the gas turbine, a fuel saturator for moisturizing fuel gas and supplying moisturized fuel gas to the fuel superheater during steady state operation and a dry fuel gas conduit for supplying dry fuel gas to the fuel superheater and the saturator, a method of controlling the supply of fuel gas to the gas turbine during startup, comprising the steps of (a) supplying dry fuel gas through the fuel superheater to the gas turbine, bypassing the fuel saturator, (b) supplying heated water to the fuel superheater to heat the fuel gas using heat recovered from the gas turbine exhaust in the heat recovery steam generator, (c) supplying heated water to the fuel saturator using heat recovered from the gas turbine exhaust in the heat recovery steam generator, (d) subsequent to step (a), admitting dry fuel gas into the fuel saturator and moisturizing the fuel gas using the heated water supplied to the fuel saturator and (e) transitioning from supplying dry fuel gas through the fuel superheater to the gas turbine to supplying heated moisturized fuel gas through the fuel superheater to the gas turbine, thereby supplying solely moisturized fuel gas to the gas turbine during premixed combustion operation.